We previously developed the method of postinjection transmission measurements for PET. This approach quantitatively accurate attenuation correction data to be calculated by appropriate correction to a "T+E" scan (transmission + emission) using the separately measured emission scan. The development of this procedure required the determination of the necessary correction factors for 1) spatially varying deadtime associated with rotating transmission sources, 2) residual emission contamination, and 3) "T+E randoms" (a coincidence between one transmission source photon and one emission photon). The accurate determination of these factors became more complex with the application of this technique to our newest scanner using block crystal design which has high detector deadtime. The purpose of this study is to validate the accuracy of this technique using phantom and patient studies. As part of a regular protocol, patients received two transmission scans, one scan before injection, and one scan 25 min postinjection of 5-10 mCi of FDG. After appropriate correction for patient motion, the transmission scans and emission scans (one emission scan corrected with both transmission scans) are compared for bias and variability. The study is currently in the phase of completion of data collection and ongoing data analysis and manuscript preparation.